The present invention relates to a ferroelectric liquid crystal light valve of a light writing type applied to an intermediate image forming medium for a printer, display device, a spatial light modulator for optical information processing, and the like.
The ferroelectric liquid crystal used as a light modulating material for the light writing type ferroelectric liquid crystal light valve can be made in a single crystalline state by allowing the thickness of the liquid crystal layer to be smaller than a helical pitch so as to eliminate the spiral as shown by Clark and Lagerwall. Ferroelectric liquid crystal has a relatively great spontaneous polarization in the uniaxial direction of the molecules, and the liquid crystal molecules are turned along a cone in a direction corresponding to the polarity of the applied voltage, causing the liquid crystal to exhibit clear bistability.
Such a light valve has heretofore been driven by the application of a bipolar voltage or a rectangular-wave voltage. That is, the light valve is driven while a bipolar pulse voltage or a rectangular-wave voltage that is lower than the threshold voltage of the light valve when dark, and greater than the threshold voltage when it is irradiated with write light carrying write information, is being applied to the light valve and read light and the write light are being projected on to the valve at all times.
FIGS. 4 show (a) and (b) a drive waveform and an optical response of the light valve based on a conventional method. The light valve is set in a reflection optical system in crossed-nicols state. A positive voltage is applied for erasing an image over the whole surface of the light valve; a negative voltage is applied for writing image. When the light valve is not irradiated with the write light, it remains nearly in a dark field state even though the negative voltage is applied thereto. When the light valve is irradiated with the write light, however, the liquid crystal molecules turn to the opposite position where the director is stable while negative voltage is being applied and stay in this position. Consequently, necessary information is written and is memorized.
However, because of the bistability which ferroelectric liquid crystal exhibites in conventional driving methods, an input image with gradation is automatically performed by threshold voltage processing and converted into a binary image. Therefore, large portion of the input information is lost, limiting the range of application.